The use of pulse-echo ranging systems for measuring, for example, the depth of a liquid in a tank is well known. In principle, the time elapsed between transmission of a pulse and receipt of an echo can be used to measure the depth of the liquid. In practice, however, a number of problems arise.
One is how to determine the “best” point in the echo profile to be used for timing. The peak of the echo is in principle where the timing should be made. However, in some cases the rising edge of the echo profile is used as it give a more stable reading than the peak. One example is where the liquid has a low dielectric constant, in which case the echo shape can be very wide and can merge with the tank bottom. Such a situation is illustrated in FIG. 1, in which the raw echo profile is shown at A. (B denotes a time-varying threshold used for suppression of false echoes; this is not directly relevant to the present application and will not be further described.) In such cases the echo peak gives poor readings that fluctuate rapidly.